“I’ve Always Wondered” has been inundated with questions about recycling. Recycling apparently both fascinates and haunts many of you, with its suspicious and menacing claim that it’s both “good for the environment” and “makes sense.” Well, fear not, gentle listener. Marketplace is here with answers.

What if you are a bad recycler and don't wash out your plastic chocolate cheesecake container?

I’m not gonna lie. It’s not pretty.

Every few days, the vast recycling machinery at the Sims Municipal Recycling plant in Brooklyn is shut down and cleaned. In part, this is because of all the vile, putrid sludge that accumulates from flakes of dried yogurt and banana peels and whatever else we put into our recycling bins because we are either lazy or overly optimistic about the power of recycling.

Perhaps more an issue is all the nonrecyclable stuff that gets put into a recycling bin (see below for a list of things recyclers have found). Worse than any chocolate cheesecake are things like plastic bags or shredded paper — they wreak havoc on recycling separation machines (normal paper is OK if the plant is designed to handle paper; and not all are).

The dirtier or more contaminated recyclable material is with nonrecyclables, the more likely the recycling plant will just throw all of it into a landfill. Nonrecyclable stuff also reduces the efficiency of the separation, which makes recycling less profitable.

In single-stream recycling, where all the recyclables are placed in the same bin, are the bags of mixed stuff sorted by people or machine?

Mostly by machine, with some human help.

At the ReCommunity Recycling plant in Beacon, New York, actual human beings scan conveyor belts loaded with recyclables before they go through various sorting machines. The humans are on the lookout for things that can damage the recycling machines or things that they would prefer to not recycle. The gems they find include:

Guns

A 6-foot section of telephone pole

A bowling ball

A bale of marijuana

A grenade

Syringes with needles

Shredded paper

Sneakers

A wedding ring

Wedding gifts

There are also a number of ingenious machines used at most recycling plants to separate items from one another. The Sims Municipal Reycling plant has 2 miles of conveyor belt moving 1,000 tons of recyclables every day from machine to machine.

A disc screen — rotating metal discs move recyclables along and smash glass bottles as it goes. The shards fall down through the machine into a waiting conveyor belt.

A ballistic separator — recyclables of different densities and shapes fall in different ways. Kind of like if you were to play with a beach ball in the rain. The rain falls to the ground, but you’re able to keep the ball in the air because it’s light and puffy. This machine plays beach ball with everything, jumbling it around. Flat things like paper and film go one way, and bulky things like containers go another.

Giant magnets – pick up magnetic metal things.

Optical sorter — As plastic moves along a conveyor belt, it moves through a strong light. A computer is photographing the moving bits and pieces, and identifies them based on the spectrum of light they absorb or reflect. Within fractions of a second, the computer tells air jets to puff certain plastics off the conveyor belt and into their own bin. Humans do some basic separation by hand as well.

Eddy current separator — A lot of people don’t realize this, but metals like aluminum and copper can interact with magnets. An incredibly powerful rare earth magnet spun at 2,900 revolutions per minute will repel them. Here’s an explanation and a video on how copper and aluminum interact with strong magnets when moving.

Does recycling make economic and environmental sense when you add everything up? Has recycling reduced the number of landfills?

It usually makes economic sense, and just about always makes environmental sense.

Environmentally it’s a no-brainer. Why dig aluminum ore out of the ground, transport it around the globe, chemically digest it and electrolyze it in furnaces, when you could just melt down some old cans? Energy savings for aluminum recycling over producing aluminum from scratch are around 95 percent. According to the EPA, recycling reduces solid waste by 49 percent, reduces net greenhouse gas emissions by 70 percent and reduces air pollutant emissions by 90 percent.

Economically, it usually makes sense. But not in every case.

Recyclables are valuable. That’s why, for example, people steal them. A lot. Aluminum, in the past, has gone for $2,000 per ton. Copper has gone for $9,000 a ton. Even paper can go for hundreds of dollars per ton. The fact that recyclables can be sold to manufacturers of bottles, clothing, carpet and cans is why recycling makes economic sense.

Recycling doesn’t cover the cost of collection for a city like New York, but it is certainly less expensive than not recycling. It costs money to send trash to a landfill or incinerator. New York pays 20 percent less for recycling than for dealing with its trash. The difference is not as large in certain areas of the Midwest, however, where landfill fees can be cheap. In those instances, the savings to a city may only come from revenue sharing from the sale of the raw materials (like compacted aluminum cans, etc).

There is a reason, for example, why we don’t often recycle plastic foam. It’s perfectly recyclable, and there are manufacturers who will buy condensed, melted down plastic foam to make lawn chairs. In fact, industrial recycling does happen, but in the consumer world, the cost of collecting, cleaning and consolidating plastic foam doesn’t make it worth it. So it’s rarely done.

Glass is increasingly falling into the same category as plastic foam. The money recycling plants get from selling their crushed glass (known as cullet) is significantly lower than what they can get for aluminum or plastic. The city of Harrisburg, Pennsylvania, recently decided it wasn’t worth it to recycle glass, and to focus on more valuable recyclables like aluminum or plastics.

Commodity prices have been falling steeply in the past year, which affects the prices recyclers can sell their metals and plastics for. In many cases they’ve come down by half, putting many recyclers in a difficult situation, and in some cases, contributing to bankruptcies and plant closures.

How much does it cost to recycle glass? How much energy and water does it take to turn a glass bottle back into a glass bottle? Is it worth it?

It takes 30 percent less energy and creates 50 percent less pollution to use recycled glass to make new glass than does making new glass from scratch.

Owens-Illinois, for example, is a massive glass producer. It melted 12.3 million tons of glass last year and created 40 billion containers (you can see how here). Thirty-eight percent of that was from recycled glass, and the company is trying to increase that number to 60 percent. The reason is that recycled glass melts at a lower temperature than the mixture of sand, lime and other ingredients often used to create new glass, so it takes significantly less energy.

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